Mixing machines for bitumen are known, comprising a cylindrical container for containing liquid bitumen, filler and solid granules to be mixed, which is developed along a vertical longitudinal axis and typically has a side air space and a bottom air space within which a serpentine is arranged, which is adapted to circulate high-temperature diathermic oil for heating the side wall and the bottom wall of the container.
Furthermore, mixing machines comprise a central auger, which is arranged within the container in a position which is coaxial to the longitudinal axis and is coupled thereto in order to freely rotate about the longitudinal axis itself; and a drive shaft which is at least partially arranged within the container in a position which is coaxial to the longitudinal axis, and is actuated by an electric motor, placed outside the container itself, in order to rotate the auger about the longitudinal axis so as to determine the mixing of the liquid bitumen with the solid granules so as to obtain a liquid bituminous mix.
More in detail, the above-described auger is structured so that the distance between its crest and the vertical longitudinal rotation axis is less than the inner diameter of the container. In use, the fluid mass present in the middle of the container is pushed downwards by the auger while being heated up by receiving heat from the bottom and side walls. The heating caused on fluid mass and solid granules determines, on one hand, a dissolution of the granules and, on the other hand, generates a convective motion which pushes the mass upwards thus determining, along with the forced motion generated by the auger, the bituminous material and dissolved granules mixing up.
The above-described mixing machines have various technical drawbacks.
Firstly, the dissolution of the solid granules may be obtained only by generating/transmitting high amounts of heat on/from the surfaces of the container, the latter being a particularly disadvantageous condition because a great amount of energy is employed for heating the diathermic oil and thus it negatively affects the total production costs of the bituminous mix.
Furthermore, the above-described mixing machine is not very efficient when mixing solid granules characterized by a particularly high specific weight. Indeed, in this case, due to their weight the solid granules tend to deposit on the bottom wall of the container, not mixing homogenously with the remaining bituminous mix.
In addition, the above-described mixing machine is not very efficient whenever solid granules characterized by a series of inner impurities having a high toughness need to be mixed. Indeed, in this case, the complete dissolution of the granule and impurities may be obtained only by considerably increasing the thermal energy and extending homogenisation times, i.e. the working times of the auger. Unfortunately, both these conditions determine prohibitive energy costs. In addition, whenever the aforesaid complete dissolution condition is not achieved, solid impurities are deposited inside the bituminous mix which causes a weakening and a thus a deterioration of the inner structure thereof.